This embodiment of the invention relates to digital signal processing, and more particularly to echo cancellation in voice processing systems such as voice mail systems and communication or audio systems.
Echo cancellers are used to reduce the amounts of undesirable echo in telephony voice mail systems. This echo comes from the system's far end as a result of playing voice prompts and is dependent on the distance between the near and the far ends, as well as on the quality of the phone line wires and the connections between them. It is not desired because it can mask a desired response coming from the far end, e.g. dual tone multiple frequency (DTMF) digits. Echo cancellers are usually built as adaptive filters, because they must adapt to the characteristics of specific channels. A digital filter is defined as an array of numbers (also referred to as “coefficients” or “taps”). The array has a length L. A signal is said to be “digitally filtered” when L values of the signal are multiplied by the L filter coefficients.
Two problems have been faced with such adaptive filters. One is the amount of echo reduction. Echo should be reduced to a level which is under a specified (desired) level of the DTMF digits, where level refers to the power of the signal. Values are usually required to be under −39 dbm. Another problem is the amount of time needed for the adaptation of the filter. Echo cancellers' tap values are usually initialized either with zeroes or with some initial values (line probing), but in order to more closely imitate the characteristics of the channel, they need some time to converge, meaning to obtain values that represent these characteristics with a higher precision. There are theoretical scenarios, where user input (DTMF digits) can be missed if the convergence time is too long.
This invention provides the needed echo reduction and reduces the time needed for adaptation of the echo-cancelling filter.